1. Field of the Invention
The invention relates to logging of subterranean formations for the determination of formation density by using gamma radiation and, more particularly, to the determination of formation density while drilling a borehole traversing the earth formation. Most particularly, the invention relates to the determination of formation density without regard to the radial position of the logging probe within the borehole or collimation of the gamma radiation employed to obtain density measurements.
2. History of the Prior Art
In the drilling of boreholes into formations in the earth, it is highly desirable to obtain information related to the nature and structure of the formation through which the borehole is passing while drilling is in progress. Being able to provide to the drilling operator information related to the characteristics of the formations while drilling is in progress enables logging of the borehole during drilling and, hence, much more efficient operation. Such measuring-while-drilling (MWD) logging either partially or totally eliminates the necessity of interrupting the drilling operation to remove the drill string from the hole in order to pass wire line logging sondes into the borehole for logging the characteristics of the formations therealong. In addition, the ability to log the characteristics of the formation through which the drill bit is passing, such as the density of the formation, greatly enhances the safety of the drilling operation. The drilling operator may thus be notified of the entry of the borehole into formations which may be likely to produce hazardous drilling conditions, such as blow out.
Heretofore, numerous techniques have been used in the wire line logging of drilled boreholes in order to determine the nature of the formations through which the borehole passes. One technique for formation density logging has included gamma ray density probes which are devices incorporating a gamma ray source and a gamma ray detector, shielded from each other to prevent the counting of gamma radiation by the detector which emanates directly from the source. During the operation of the probe, gamma rays (photons) are emitted from the source and enter the formation to be studied. In the formation they interact with the atomic electrons of the material of the formation by either photoelectric absorption, by Compton Scattering, or by pair production. In both photoelectric absorption and pair production phenomena, the particular photons involved in the interaction process are removed from the gamma ray beams.
In the Compton Scattering process, the involved photon loses some of its energy while changing its original direction of travel, the loss of energy being a function of the scattering angle. Some of the photons emitted from the source into the formation material are accordingly scattered back toward the detector. Many scattered rays do not reach the detector, since their direction is again changed by a second Compton Scattering, or they are thereafter absorbed by the photoelectric absorption process or the pair production process. The scattered photons which reach the detector and interact with it, are counted by electronic counting equipment associated with the detector.
Major difficulties encountered in conventional gamma ray density measurement include rigorous definition of the sample size and the limited effective depth and sampling times. Other major difficulties include the disturbing effects of undesired interferring materials located between the density probe and the formation sample, such as drilling mud and mud cake on the borehole wall, which have required that the probe be positioned directly against the borehole wall.
Numerous prior art wire line gamma radiation logging probes have tried to compensate for the effect on formation density measurements produced by the density of the mudcake on the walls of the borehole by providing two detectors axially spaced along the borehole at different distances from the source of radiation. The near, or short spaced detector is for receiving radiation which has scattered mainly in the materials near the borehole wall, and therefore in the mudcake. The far, or longspaced detector is for receiving radiation which has scattered principally in the formation.
Most prior art gamma logging systems have required complex collimation schemes to narrowly define either the beam of radiation emanating from the source to direct it into a specific region of the formation or the beam of radiation received back by the detector to insure that only radiation back-scattered from a particular region of the formation was detected, or both. In addition, prior art wire line gamma ray logging sondes have been highly susceptible to variation in density measurements due to the thickness of the drilling mud as well as the mud cake on the walls of the borehole through which the radiation must pass and, thus, the accuracy of the measurements is strongly affected by the eccentricity of the tool within the borehole. For this reason, prior art tools include elaborate mechanisms for pressing the surface of the tool firmly against the wall of the borehole on the side of the borehole at which point measurement is being made.
Needless to say the difficulties encountered in prior art wire line gamma radiation logging would be further complicated if the density measurement tool is made part of a drill string and operated during drilling of the borehole. The only known gamma radiation formation density probe useful in measurement while drilling apparatus is shown in U.S. patent application Ser. No. 478,979, filed Mar. 25, 1983, now abandoned, by Daniel Coope, entitled Formation Density Logging While Drilling, and assigned to the assignee of the present invention. This application discloses a technique for gamma-gamma formation density logging while drilling which relies upon the collimation of gamma radiation and a pair of axially spaced detectors along the borehole from one another and from the source of radiation in order to examine radiation back-scattered from two different regions within the formation at different distances from the walls of the borehole.
One prior art wire line density probe which functions regardless of the thickness and the chemical composition of materials that are located between the density probe and the sample is shown in U.S. Pat. No. 3,846,631. This technique comprises passing two gamma ray beams from two intermittently operated gray sources into the sample, receiving the radiation back scattered from each of the two sources by two separate detectors, and building ratios of products of the four separate counting rates in such a manner that the numerical result is an indication of the density of the sample. In such longitudinally spaced two detector probes, which must be deployed against the borehole wall, the spacing between the detectors is a critical dimension. If the interferring formation materials are non-uniform over distances comparable to the spacing of the two detectors, the measured density will be erroneous.
In gamma radiation formation density probes, which include collimation of the gamma radiation, it is presupposed that the region of interaction between the radiation and the formation can be narrowly defined and restricted to a small region. Not only is precise collimation of gamma radiation beams difficult to accomplish, but the assumption that a collimated beam only interacts with a precisely definable portion of the formation surrounding the borehole is erroneous.
It would be an advantage therefore to overcome the limitations and inaccuracies of the prior art through a system for measuring the density of subterranean formations while drilling a borehole traversing the formation without the necessity of defining a narrow region of interrelation of gamma radiation with the formation or the employment of radiation collimation, or the necessity of tool deployment against the borehole wall.
The present invention provides such a system through novel geometry of a gamma radiation source and detectors which enables measurement of formation parameters from back-scattered gamma radiation without regard to the eccentric position of the tool within the borehole. The measurement is also made without regard to any assumption as to the particular region of the formation from which the radiation was back-scattered.